Anna,
The lack of battering inside the frame is not surprising, since under normal operation the breechblock never gets that far back--the toggle tail impingement on the rear of the frame is the normal stop and in the case of this gun is heavily battered, as you point out. I wonder if there is any evidence on the front of the connecting stirrup of contact with the breechblock?
I'm not sure that the mess on the recoil lug as shown is due to recoil impact. What does it's rear face look like?
The presence of the toggle lock in the toggle indicates that the screw inserted to retain the center toggle pin is just an additional precaution--"belt and suspenders", as it were. I wonder if there was evidence of its movement during firing which prompted the addition?
As I look at these pictures this morning I see another anomaly with this gun, which I merely observe here due to lack of time for consideration.
On a model 1900, the top surface of the rear toggle is even with the top surface of the rear of the receiver rails. As pointed out here, the rear toggle flat is raised above the receiver railsl, as is the practice with 1906 and later guns.
--Dwight